The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as blood vessel, to maintain the patency thereof. These devices are useful in the treatment of atherosclerotic stenosis in blood vessels.
Stents are generally tubular-shaped devices which function to hold open a segment of a blood vessel, coronary artery, or other anatomical lumen. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway therethrough.
Further details of prior art stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 4,856,516 (Hillstead); U.S. Pat. No. 4,886,062 (Wiktor); U.S. Pat. No. 6,066,167 (Lau et al.); and U.S. Pat. No. B1 5,421,955 (Lau et al.), which are incorporated herein in their entirety by reference thereto.
Various means have been described to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location includes mounting the expandable stent on an expandable member, such as a balloon, provided on the distal end of an intravascular catheter, advancing the catheter to the desired location within the patient""s body lumen, inflating the balloon on the catheter to expand the stent into a permanent expanded condition and then deflating the balloon and removing the catheter. One of the difficulties encountered using prior stents involved maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery. Once the stent is mounted on the balloon portion of the catheter, it is often delivered through tortuous vessels, including tortuous coronary arteries. The stent must have numerous properties and characteristics, including a high degree of flexibility in order to appropriately navigate the tortuous coronary arteries. This flexibility must be balanced against other features including radial strength once the stent has been expanded and implanted in the artery. While other numerous prior art stents have had sufficient radial strength to hold open and maintain the patency of a coronary artery, they have lacked the flexibility required to easily navigate tortuous vessels without damaging the vessels during delivery.
Generally speaking, most prior art intravascular stents are formed from a metal such as stainless steel, which is balloon expandable and plastically deforms upon expansion to hold open a vessel. The component parts of these types of stents typically are all formed of the same type of metal, i.e., stainless steel. Other types of prior art stents may be formed from a polymer, but again, all of the component parts are formed from the same polymer material. These types of stents, the ones formed from a metal and the ones formed from a polymer, each have advantages and disadvantages. One of the advantages of the metallic stents is their high radial strength once expanded and implanted in the vessel. A disadvantage may be that the metallic stent lacks flexibility which is important during the delivery of the stent to the target site. With respect to polymer stents, they may have a tendency to be quite flexible and are advantageous for use during delivery through tortuous vessels. On the other hand, such polymer stents may lack the radial strength necessary to adequately support the lumen once implanted.
What has been needed and heretofore unavailable is a stent that has a high degree of flexibility so that it can be advanced through the tortuous passageways of a patient and can be readily expanded, yet have the mechanical strength to hold open the body lumen into which it expanded. The present invention satisfied this need.
The present invention in one exemplary embodiment is directed to an intravascular stent comprising a plurality of metallic rings expandable in a radial direction, wherein each of the rings is aligned on a common longitudinal axis, and each of the rings has at least one formation therein; at least one flexible link formed of a polymer having a proximal end and a distal end, with a length that spans at least two rings; and wherein the link has sufficient column strength to axially separate the rings, and wherein the link interconnects the rings at the formations.
As a result, the present invention expandable stent is relatively flexible along its longitudinal axis to facilitate delivery through tortuous body lumens, but is stiff and stable enough radially in an expanded condition to maintain the patency of a body lumen such as an artery when implanted therein. The resulting stent structure is a series of radially expandable cylindrical rings that are spaced longitudinally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the lumenal wall, but not so close as to compromise the longitudinal flexibility of the stent.
The rings are attached to each other by flexible links such that at least one flexible link attaches adjacent rings. If desired, more than one link can be used to attach adjacent rings. At least some of the links are formed from a polymeric material that provides flexibility to the link and allows the stent to more easily bend or flex along its longitudinal axis as the stent navigates through tortuous vessels or coronary arteries. The flexibility of the links is balanced against the links having sufficient column strength to properly orient and separate the rings along the stent longitudinal axis so that the rings do not telescope into each other, collapse longitudinally, or overlap one another. The combination of the flexible rings and flexible links cumulatively provide a stent that is flexible along its length and about its longitudinal axis, yet is still relatively stiff in the radial direction after it has been expanded in order to maintain the patency of a vessel and to resist collapse.
The stent embodying features of the invention can be readily delivered to the desired body lumen, such as a coronary artery (peripheral vessels, bile ducts, etc.), by mounting the stent on an expandable member of a delivery catheter, for example a balloon, and advancing the catheter and stent assembly through the body lumen to the target site. Generally, the stent is compressed or crimped onto the balloon portion of the catheter so that the stent does not move longitudinally relative to the balloon portion of the catheter during delivery through the arteries, and during expansion of the stent at the target site. In a self-expanding embodiment of the present invention stent, the inflation balloon may be omitted. Such self-expanding stents have metallic rings that are made of nickel-titanium or like shape memory alloys.
Stent deliverability and stent flexibility are improved with the present invention hybrid stent. However, one challenge facing the hybrid stent is the potential for vessel injury caused by the strut or link surface presented to the vessel wall. If the polymer links lie on top of the rings or otherwise protrude outwards, the links might contact the vessel wall upon balloon or self-expansion. This expansion creates anon-uniform force against the vessel wall potentially causing neointimal formation. With greater neointimal formation, there is a higher probability of vessel injury. Also, the junction of a polymer link with a metallic ring might present a larger thickness than the rest of the stent, which could lead to greater restenosis at those points.
To address some of the above-mentioned issues, the present invention stent includes formations at the rings to which the polymer links are joined, attached, bonded, or otherwise secured. These formations could be, for example, a hole, a notch, a groove, or the like. The polymer link is threaded through the hole or passes over the notch or groove. Heat can then be applied to melt the polymer link to the ring. It is also possible to partially or fully encapsulate the junction at which the link meets the ring.
Alternatively, the polymer link may be mechanically locked to the ring. This can be accomplished by wrapping the link around the formation and then optionally heating and welding the polymer to the ring. In yet other embodiments, the polymer link can be threaded though a hole in the ring and beads formed in the polymer on either side of the ring by applying beat. The ring is thus locked in between the beads.
By use of the formations in the rings, it is possible to minimize the profile of the junction where the link meets the ring. Accordingly, any potential trauma to the vessel wall brought about by this junction is minimized.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying exemplary drawings.